This invention is directed to a simple method of forming a hydrophobic coating on an aluminum surface.
Many processes are presently available for coating aluminum surfaces, such as anodizing, plating, chemical conversion coatings, painting and the like. The coatings, although designed for long life, require extensive surface pretreatments and are quite expensive. However, frequently, only short-term protection is need or desired, for example, in shipping or storing semi-fabricated aluminum products, such as coiled sheet and the like, to prevent the formation of water stain or other oxidation products. On other occasions, it is desirable to prevent the gradual buildup of natural oxide on the aluminum surface, for example, in welding applications and adhesive bonding applications because the buildup of natural oxide can interfere with these types of operations. However, no simple and inexpensive process is presently known which will give short-term protection without interfering with subsequent fabrication or surface treatments, particularly when lubricants must be applied to the surface.
A simple, inexpensive process for coating aluminum is described by Wittrock et al. in U.S. Pat. No. 3,726,721, assigned to the present assignee, but the friable coating formed by this process can interfere with subsequent forming or coating operations.
Kubie in U.S. Pat. No. 2,963,391 describes a process for forming a coating designed as an extrusion lubricant wherein the aluminum surface is first treated with an ammonia-laden alkaline solution containing a fatty acid (or equivalent salt or ester thereof) and then baked at about 400.degree. F to form a coating having unknown properties except for lubrication.
Marosi in U.S. Pat. No. 3,849,207 describes and claims a process for treating aluminum and other metals wherein the surface is treated with an alkaline sodium formate solution and then coated with a clear resinous film to form a sepia-colored coating. The nature of the coating formed during treatment in the alkaline sodium formate solution is not described in the reference. However, it has been found that the aluminum surface underlying such a coating is susceptible to water stain or other oxidation in much the same manner as untreated aluminum because the coating is readily wet and penetrated by water or aqueous solutions.
A simple, inexpensive method to temporarily prevent extensive oxidation of an aluminum surface by water or other media particularly in coiled sheet has been needed for many years, but has been heretofore unavailable.
It is against this background that the present invention was developed.